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Galaxies come in all shapes and sizes. Some just look like blobs of stars; we call these elliptical galaxies. Others, including our own Milky Way, have a bright centre with arms of stars spiralling out; we call these spiral galaxies. There are also galaxies that don’t fit into either of these categories and these are known as irregular galaxies. Examples of galaxies are shown below in Hubble Space Telescope pictures (credit: ESA/Hubble).

It is believed that all galaxies have a “supermassive black hole” (SMBH) at their centres. Supermassive means that these black holes have masses millions or billions of times that of the Sun. In most galaxies, like the Milky Way, this SMBH is pretty boring. The Milky Way’s SMBH is called Sagittarius A* and we can tell that it is there by looking at how stars move around in the very centre of the galaxy (there’s a cool animation of this on the Max-Planck-Institut für extraterrestrische Physik website) but apart from this, the SMBH doesn’t do much to announce that it exists. However, there are some galaxies that have centres that are really bright, too bright to be explained by adding up the light from all of the stars in the galaxy. These objects are called Active Galactic Nuclei (AGN) and it is thought that they are this way because the central SMBH is in a “active” stage, pulling surrounding material into it through gravity and spewing out loads of energy, not just as visible light that we can see, but all across the electromagnetic spectrum.